Category Archives: Infectious Diseases

Infectious Diseases

Zika Virus Testing in Pregnant Women: New CDC Recommendations

Zika3 figureWe have previously blogged about testing for Zika virus in pregnant women here and here. But testing recommendations continue to evolve.

As a result of increasing knowledge about the Zika virus, the CDC updated their recommendations.   The changes are based on declining trends in the number of reported cases of Zika virus infection in the Americas, emerging evidence on prolonged detection of Zika IgM antibodies, and new limitations for interpreting serologic tests during pregnancy. IgM is most likely to be detected in the first 12 weeks after infection but may persist beyond 12 weeks in some infected individuals, limiting the ability of testing to determine whether an infection occurred during or prior to pregnancy. False positive results and cross-reactivity with other flaviviruses can occur with IgM assays. Therefore, it is important to ascertain whether a woman had exposure to flaviviruses other than Zika virus before the current pregnancy because a positive IgM result might have been caused by cross-reactivity from a previous flavivirus exposure. Given the possibility of a false positive result, laboratory test results should not be released until all testing is complete.

Zika virus tests should be performed in:

  • Symptomatic pregnant women with possible exposure to Zika virus
  • Asymptomatic pregnant women with ongoing possible exposure to Zika virus
  • Pregnant women with possible exposure to Zika virus who have a fetus with prenatal ultrasound findings consistent with congenital Zika virus infection
  • Non-pregnant symptomatic individuals with possible exposure to areas with risk of Zika virus transmission

Zika virus testing may be considered for:

  • Asymptomatic pregnant women with recent possible but no ongoing exposure to Zika virus (i.e., travelers). Although not routinely recommended, testing may be considered on a case-by-case basis and in line with jurisdictional recommendations.

Zika virus testing is not recommended for:

  • Non-pregnant asymptomatic individuals
  • Pre-conception screening

The primary specimens for Zika virus testing should be paired serum and urine samples. Some emergency authorization tests may require other samples types. However, serum should always be obtained in case nucleic acid testing (NAT) testing is indicated after initial test results. NAT (on serum and urine) and IgM serological testing should be performed concurrently when testing symptomatic pregnant women. Specific guidance for laboratories is given by the CDC.

An Update on Zika Virus Testing in Pregnant Women

We have previously blogged about Zika virus   during pregnancy.That post was in February of 2016 and a lot has changed since then.


For one thing, Zika has been shown to be transmitted via sexual contact from an infected individual (even if he or she does not have symptoms) to a non-infected individual.  The use of condoms can help, and there are guidelines for the pre-conception prevention of sexual transmission. 

In addition, scientists at the CDC have concluded that Zika virus infection does cause microcephaly and other brain defects. This means that women who are infected with Zika virus during pregnancy are at increased risk of having a baby with these problems, but it does not mean that all women with Zika virus infection during pregnancy will have these problems.

Other changes that have occurred since last year include the FDA's Emergency Use Authorization (EUA) for several diagnostic tools for Zika virus, including the Trioplex Real-Time RT-PCR (rRT-PCR) assay and the Zika MAC-ELISA. 

The Trioplex assay is for the detection of RNA from dengue, chikungunya and Zika viruses in serum, whole blood (EDTA), and cerebrospinal fluid (CSF). This is important since dengue and chikungunya are often in the same differential diagnosis with Zika virus. The assay can also be used to detect Zika virus RNA in urine & amniotic fluid.

The Zika MAC-ELISA is intended for the qualitative detection of Zika virus IgM antibodies in human sera or cerebrospinal fluid (CSF).  however, due to cross-reaction with other flaviviruses and possible nonspecific reactivity, results may be difficult to interpret. Consequently, presumed positive, equivocal, or inconclusive tests must be confirmed by plaque-reduction neutralization testing (PRNT).

According to the CDC, who should be tested?

Asymptomatic Pregnant Women

For asymptomatic pregnant women who have traveled to areas with active Zika virus transmission, RNA nucleic acid testing (NAT) testing is recommended on serum and urine within 2 weeks of the date of last possible exposure. Zika virus-specific IgM testing should be performed on women within 2-12 weeks after travel to an area of active transmission or who have had sexual contact with a man confirmed to have Zika virus infection. In areas with active Zika virus transmission, asymptomatic pregnant women should undergo IgM testing as part of routine obstetric care in the 1st and 2nd trimesters. Presumed positive, equivocal, or inconclusive IgM results must be confirmed by plaque reduction neutralization test (PRNT).

Symptomatic Pregnant Women

For symptomatic pregnant women with exposure to Zika virus, RNA nucleic acid testing (NAT) of serum and urine is recommended up to 2 weeks after symptom onset. Whole blood can also be tested for Zika RNA alongside serum and urine. Urine should always be collected with a patient-matched serum specimen. A positive RNA NAT result on any sample confirms Zika virus infection and no additional testing is indicated. A negative RNA NAT result does not exclude Zika virus infection and serum should be tested for the presence of IgM antibodies. If more than 2 weeks have passed since the onset of Zika virus symptoms, specific IgM testing is recommended. Reflex RNA NAT testing should be performed as a subsequent test for pregnant women who are IgM positive.

For recommendations of testing non-pregnant women, and infants, visit the FDA website.

Zika Virus Testing in Pregnant Women

Zika Figure croppedZika virus is a mosquito borne illness that is found in South and Central America. The most common symptoms include fever, rash, joint pain, and conjunctivitis (red eyes). The virus is spread by mosquitos primarily in the Aedes aegypti and Aedes albopictus species which also carry other tropical diseases such as Chikungunya and Dengue. These mosquitos bite humans primarily in the daytime. It is estimated that 80% of people infected with the Zika virus are asymptomatic. In most people with symptoms, the illness is self-limited and resolves in 5-7 days. Disease requiring hospitalization is rare.

Recently, there have been reports in Brazil of an increased rate of microcephaly and other poor pregnancy outcomes in babies from women who were infected with the Zika virus while pregnant.  However, further studies are needed to understand the relationship between these outcomes and infection. In the meantime, the Centers for Disease Control and Prevention (CDC) have issued special travel precautions for pregnant women and women trying to get pregnant.

So who should be tested for Zika virus and what testing is available?

Initially, the CDC advised that a pregnant woman should only be tested if she has symptoms of Zika virus within the first week of being in an endemic area. If the mother is positive, then the infant should be tested for congenital infection.

However, very recently, the CDC updated the guidelines to include asymptomatic pregnant women who live in or have traveled to endemic areas.  The update recommends that serologic testing be offered to pregnant women can be offered testing within 2-12 weeks after they return from travel. For asymptomatic pregnant women who live in endemic areas, testing is recommended at the initiation of prenatal care with follow-up testing mid-second trimester.

For infants that have microcephaly or intracranial calcifications detected prenatally or at birth with a mother who was potentially infected with Zika virus during pregnancy, the infant should be tested. If infants have positive or inconclusive test findings, the case should be reported to the State or local health department for follow-up. If the infant tests negative, other possible etiologies for the microcephaly should be investigated.

For infants without microcephaly or intracranial calcifications with a mother who was potentially infected with Zika virus during pregnancy, subsequent evaluation depends on the mother's results. If the mother test's negative, no further testing is required. If the mother received positive or intermediate results, then the infant should be tested. If the infant test's negative, then no further testing is required. If the infant test's positive then further clinical evaluation (including comprehensive physical exam, cranial ultrasound and ophthalmologic evaluation) should be performed and the infant should be followed for long term sequelae.

No commercial tests are yet available for Zika virus. Testing is performed at the CDC and some local health departments. The tests currently performed include RT-PCR, ELISA for IgM and a plaque reduction neutralization test (PRNT).

Infants who are being evaluated should have RT-PCR performed on serum (from infant or umbilical cord) within 2 days of birth. CSF if available should also be tested by RT-PCR. ELISA for IgM should be performed on infant serum and CSF.

Mothers being evaluated should have serum tested using ELISA. RT-PCR can be performed during the first week of viral infection. Amniocentesis should be offered to pregnant women who test positive or indeterminate and RT-PCR should be performed on the amniotic fluid.

Note that false positives can occur in the ELISA assay due to cross reactivity with other related flaviviruses such as dengue or yellow fever. PRNT can be used to distinguish false positives from true positive results. If neutralizing antibody titers are ≥ 4-fold greater than dengue virus neutralizing antibody titers, then Zika virus is considered positive. Immunohistochemistry can also be performed on fixed placenta or umbilical cord tissue. If any of any of the tests are positive, the infant is considered congenitally infected.

Currently, there is no anti-viral treatment or vaccination for Zika virus. Treatment is supportive. The best defense against Zika is preventing maternal infection by avoiding mosquito bites. It is important to note that, when used as instructed, insect repellants containing DEET, picardin, and IR3535 are safe for pregnant women.

Syphilis Testing

SyphilisRecently, I blogged about TORCH testing  (sometimes called TORCHES).  The "S" in TORCHES stands for Syphilis. Syphilis is a sexually transmitted infection caused by the spirochete bacterium Treponema pallidum. Syphilis can present in one of four different stages: primary, secondary, latent, and tertiary.  Many people infected with syphilis are asymptomatic, but can be at risk for complications if the infection is not treated. Syphilis is transmitted from person to person through direct contact with a syphilis lesion or sore.  These lesions occur most frequently on the vagina, anus, rectum or external genitals. The bacteria may also be transmitted from mother to fetus during pregnancy or at birth, resulting in congenital syphilis which can cause stillbirth or infant death shortly after birth. Untreated babies may become developmentally delayed, have malformations, seizures, or die. Here I'll discuss further syphilis testing.

The CDC 2010 "Sexually Transmitted Diseases Treatment Guidelines"  recommend that pregnant women be screened on their first prenatal visit for sexually transmitted diseases such as Syphilis.

Blood tests for syphilis are divided into nontreponemal and treponemal tests.

Nontreponemal tests are used for the screening of pregnant and non-pregnant people, and include venereal disease research laboratory (VDRL) and rapid plasma reagin (RPR) tests. These tests do not detect antibodies against the actual bacterium, but rather for antibodies against substances released by infected cells when they are damaged by T. pallidum (e.g. cardiolipin). Nontreponemal tests are inexpensive, relatively rapid to perform, and widely available. They can detect an active infection and will become negative over time with proper treatment, therefore, they can be used to follow therapy.  The diagnostic sensitivity of these tests varies between 80-90% for detecting primary infection, and their specificity is approximately 98%. However, because there are occasionally false positive results, confirmation of all positive nontreponemal test results is required. This is accomplished by using a more specific treponemal test.

Treponemal tests detect antibodies that are specific to T. pallidum. These tests include treponema pallidum particle agglutination (TP-PA) or fluorescent treponemal antibody absorption test (FTA-Abs). These tests cannot distinguish between past and current infection because the antibodies are generally present for life. Treponemal tests have diagnostic sensitivities of about 84-88% for detecting primary infection and close to 100% for detecting later infections. They also have specificities >95%. However, testing is time and labor intensive.

Syphilis is often diagnosed by using a nontreponemal screening test first and then performing treponemal tests on any positive samples. However the interpretation of a nontreponemal test is subjective and so this approach does not allow for high test throughput. In the last few years, a number of automated treponemal enzyme immunoassays (EIA) have been developed for the diagnosis of syphilis. A recent paper compares the clinical utility of seven of these assays.   The authors of this study concluded that the automated treponemal assays have sensitivities and specificities comparable to the FTA-Abs test (considered to be the gold standard) and a consensus panel of treponemal tests resulted in an overall concordance of 95-99%. They also concluded that the cost and turnaround time of the automated assays are comparable to the FTA-Abs test with the benefit of less hands-on time and more objective results.  This would be especially true for large laboratories that process a large number of samples.

Recently, the Centers for Disease Control and Prevention (CDC) and the Association of Public Health Laboratories (APHL) convened an expert panel to evaluate available information and produce recommendations that will ultimately result in the development of Guidelines for the Laboratory Diagnosis of syphilis testing in the U.S. the Meeting Summary Report are currently available and provide a summary of what was discussed.  It is anticipated that the formal Testing Guidelines will be released in 2011.  The summary states that in areas with a low prevalence of syphilis, samples may be screened using a treponemal-specific EIA and those with positive results analyzed with a nontreponemal test to assess for an active disease state and treatment status. This is the reverse of what is currently practiced.  

However, there is a potential problem with this new algorithm. The use of a treponemal test (FTA-Abs or treponemal EIA) for screening purposes can result in patients who are positive by a treponemal-specific screening test yet are negative by nontreponemal tests. This can occur in patients with past or recently treated syphilis and in patients with very early or late/latent disease. Therefore, physicians must review patient disease and treatment histories in order to properly interpret syphilis testing results. In addition follow-up FTA-Abs testing may be required. Several publications have examined the "reverse syphilis algorithm" in practice.

So what is the current thought? Currently, the CDC still recommends using a nontreponmeal screening test and treponemal test as confirmation of syphilis. The traditional algorithm is designed to identify patients with active syphilis while minimizing false-positive results in populations with a low prevalence of syphilis. Although the reverse syphilis algorithm has some attractive features, such as automation of testing, objective results, and detection of latent syphilis, it may not be suitable in all clinical situations.

What is TORCH Testing?

  TorchTORCH testing (sometimes called TORCHES testing) includes tests for a group of infectious diseases that can infect pregnant women and cause birth defects or death in their infants. TORCHES is an acronym for the following infectious diseases:

Toxoplasma gondii (toxoplasmosis)- a parasite that can be acquired from ingesting cysts from the feces of infected cats, drinking unpasteurized milk, or eating undercooked contaminated meat. Infection early in pregnancy can cause miscarriage. Later in pregnancy it can cause eye infections, and mental retardation.

Other– Other infections that may be screened for at the same time include Parvovirus B19 and sometimes varicella zoster virus (chicken pox). 

Rubella (German Measles)-Infection early in pregnancy can cause birth defects such as heart disease, growth retardation, and eye defects. It can also cause problems later in childhood such as hearing loss. Following the introduction of the vaccine in the 1970s, the incidence of Rubella has now dropped to approximately 1 in 10,000 births.

Cytomegalovirus (CMV)- This virus is transmitted through body secretions (including breast milk) as well as sexual contact. Infection can cause death, hearing loss and mental retardation.

Herpes simplex virus (HSV)- is a common infection that is spread by oral and genital contact. Most infections are spread to infants during the birth process. Infected infants may have localized infections of the mouth, eyes or skin, and some may have disseminated infection. Infant mortality from neonatal infection can be very high.

Syphilis (Treponema pallidum) – this bacterial infection can cause stillbirth or infant death shortly after birth. Untreated babies may become developmentally delayed, have malformations, seizures, or die.

In practice, TORCH testing in the United States is most commonly targeted toward high-risk groups, or women from areas where the prevalence of the diseases is high. In these situations, the screening serves to identify women with active infection as well as those who lack immunity to the diseases.  Those who aren’t immune can be vaccinated or more specifically counseled to limit risk of exposure.

The use of TORCH testing to diagnose these infections is becoming less common since more specific and sensitive tests, that don't rely on the detection of antibodies are available. Note that false positive results are possible and all positive TORCH tests should be followed-up with more specific confirmatory tests.  

Routine screening of pregnant women for underlying infectious disease or immunity is consistently performed to identify:

Chronic carriers of hepatitis B virus
HIV infection
Group B streptococcal colonization
Immunity to Rubella virus

In fact, the CDC 2010 "Sexually Transmitted Diseases Treatment Guidelines" recommend that pregnant women be screened on their first prenatal visit for sexually transmitted diseases which may include Hepatitis B, HIV, Syphilis as well as Gonorrhea & Chlamydia.

In addition to being tested for sexually transmitted diseases, the CDC also has a list of tips on how to avoid infections during pregnancy.

We will discuss testing for each of these infectious diseases individually in future blogs, so stay tuned!


Screening tests for group B strep infection

StreptococcusThe most common cause of life-threatening infections in newborns comes from a bacteria known as Streptococcus agalactiae (more commonly referred to as group B streptococcus or GBS).  This was stressed in a recent meta-analysis that reported that GBS infection remains an important, global cause of infant mortality.

The overall infection rate was 0.53 per 1,000 live births and, on average, about 10% of infected infants died.  Infants born in Africa were more likely to be infected (1.21 per 1,000) and die (22%) from the infection than infants born in the Americas or Europe (0.67-0.57 per 1,000 with 11 and 7% fatality rates).

It doesn't have to be this way because GBS infection is treatable with antibiotic therapy.  Indeed, in more developed countries, therapy is provided to women who carry the bacteria which prevents their baby becoming infected during delivery.  However, in poorer countries this is less likely to happen due to fewer resources.

Providing therapy to every pregnant women is not practical because not all women are colonized with GBS and so a key preventative strategy is to identify those women who do carry the bacteria.  The most sensitive test is culture performed on samples collected from the vagina and rectum.  The Centers for Disease Control and Prevention (CDC) published guidelines in 2010 that called for the routine GBS screening in all pregnant women at 35 to 37 weeks of gestation.  Testing needs to happen close to delivery (normally at ~40 weeks) because women can be colonized with GBS at anytime.  That is, a negative test result obtained earlier in pregnancy wouldn't rule-out the possibility that colonization then occured sometime after testing.  Women with a positive culture are treated with antibiotics during labor to prevent the transmission of GBS to their infant.

Although culture is considered the gold standard test for GBS screening, it is not perfect because some infants born to culture-negative women still get infected with GBS.  Also, culture techniques give results in 1–3 days, a time frame that may not be useful should an expectant mother go into labor prior to having the culture test performed.  For these women, DNA-based tests can be used.

These tests detect the presence of GBS using a DNA amplification technique like PCR and give results in a few hours rather than days.  Currently, these types of tests are not as sensitive as culture (i.e. they can give false-negative results) and so they aren't recommended for routine screening of women who are not in labor.  Their sensitivity is improved by using an enriched sample (one where the bacteria are allowed some time to multiply in a growth media), the use of this type of sample is impractical for women in labor when results are needed quickly.

Until an effective vaccine to prevent GBS infection is available, laboratory testing will remain an essential tool for identifying and preventing GBS.